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The Forefront of Space Science

Source of Solar Wind Revealed by HINODE
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Solar wind means literally “windEblown from the Sun. It is different from the wind blowing around us on earth. In fact, solar wind is a high-speed stream of charged particles such as electrons or protons. Its velocity reaches up to 200km to 800km per second around the earth. The wind blows beyond earth and the solar system to finally reach about 15 billion km from the Sun (100 times the distance between the Sun and earth), where it forms a spherical termination shock plane in the interstellar gas, the center of which is on the Sun. It was reported that NASA's spacecraft Voyager 1 and 2 launched in 1977 passed the termination shock plane (Voyager 1 in 2004, Voyager 2 in 2007) after completing their observations of Jupiter, Saturn, etc. The orbital radius of Neptune, the farthest planet in the solar system, is 4.5 billion km, so it can be seen that solar wind envelops the whole solar system. The wind has various effects on us. Solar wind explains why the shape of the earth’s magnetosphere is like a streamer in the opposite direction of the Sun and why aurorae shine on the earth, Jupiter, Saturn, etc. Since the solar wind carries charged particles, it sometimes causes charging troubles on satellites orbiting the earth. In today’s society where satellites play important roles in social supports such as weather forecasting, communications and GPS, it can be said that solar wind has started to affect our daily life.

However, we lack sufficient knowledge about from where and how the wind blows. There are two types of solar wind, fast and slow. With the observation by the Ulysses satellite launched in 1990, it was verified that the fast wind comes mainly from the high-latitude regions of the Sun including its Arctic and Antarctic regions, while the slow wind comes from the low-latitude region. Until now there has been no observation that has directly imaged the site of solar wind’s outflow in corona. In addition, we lack an understanding of the mechanism by which the wind leaves the Sun, or how it is accelerated up to the speed that it blows in interplanetary space (i.e., supersonic speed).

X-ray telescope of HINODE

Solar-observation satellite HINODE was launched in September 2006. It carries three of the most advanced telescopes: Solar Optical Telescope (SOT); X-ray Telescope (XRT); and Extreme-Ultraviolet Imaging Spectrometer (EIS). Let me explain about the XRT, which plays an important role in the theme of this article. XRT can identify 2-D distribution of temperature and density of gas in the corona (N.B. gas becomes plasma due to high-temperature environment). The telescope can also trace the profile of magnetic lines in the corona through its X-ray images because ionized plasma gas moves only along the magnetic lines. XRT adopts the same optical system of grazing incidence as the SXT (soft X-ray telescope) onboard YOHKOH, a solar-observation satellite that preceded HINODE. By increasing its size, however, the XRT’s spatial resolution is improved to about three times that of SXT. Further, by employing the back-illumination-type CCD as a focal-plane detector, the telescope is able to observe the relatively low-temperature corona at 1 to 2 million deg. C., something SXT was unable to detect due to its low sensitivity. Because of the satellite’s increased data-processing and transmission speeds, when observing quiet Sun without the occurrence of flares, XRT is able to take images much more frequently (by one order of magnitude) compared to SXT. With its more-frequent imaging observation of the low-temperature corona, it has started to discover previously unknown, interesting phenomena in the corona. One such phenomenon is the outflow of coronal gas, the origin of the solar wind discussed in this article.

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